Under normal conditions, hydraulic cements, such as Portland cement, quickly develop compressive strength upon introduction to a subterranean formation, typically within 48 hours from introduction. As time progresses, the cement develops greater strength while hydration continues. Depending on temperature, hydration is nearly complete and compressive strength increases become insignificant after a year from introduction.
At temperatures above 230° F., however, the cement reaches most of its final strength within the first few weeks from introduction into the formation. After that point, the cement starts to deteriorate, loses strength and gains permeability, a phenomenon known as strength retrogression. As the temperature further increases, strength retrogression becomes more severe. At about 450° F., the peak value of compressive strength is reached in less than 24 hours and subsequently declines. Further, the peak compressive strength at this temperature is much lower than the compressive strength developed at lower temperature conditions.
Strength deterioration is mainly caused by a change in the structure of the cement which, in turn, is created by chemical reactions. Such reactions are dependent on temperature and result in the conversion of calcium silicate hydrate and excess lime into alpha-dicalcium silicate hydrate, a mineral that has a lower bulk volume and higher porosity. Alpha-dicalcium silicate hydrates are weaker than calcium silicate hydrates.
Increasing the mix water ratio, often required to maintain slurry mixability, typically causes more severe strength retrogression since the resulting cement typically is more porous and permeable than heavier cement systems. To prevent strength retrogression in Portland cements, powder silica or silicon dioxide (sand) has been added in sufficiently high concentrations to render a Si:Ca ratio higher than 1. Even though silica sand is considered inert at ambient temperatures, when mixed with cement, it reacts with cement at temperatures above 250° F. to produce monocalcium silicate hydrates. These hydrates are less porous and stronger than alpha dicalcium silicate hydrate.
The minimum concentration of silica by weight of cement (BWOC) required to obtain a reasonable Si:Ca ratio is 20% and the preferred concentration is 35%, although many geothermal formulations require higher concentrations, up to 50% BWOC. In low-density cements prepared with mix water ratios higher than 55%, greater silica is required to stabilize the cement at higher temperatures. In some instances, 40 to 50% BWOC is required.
Silica fineness is also an important attribute. The use of fine silica produces brittle and harder cements, often with higher compressive strength. Coarser silica produces ductile and softer cements with lower compressive strength.
Interest in more flexible cements has increased during the last few years since such cements provide better zonal isolation in oil and gas wells; harder cements crack easily with cyclic stresses caused by pressure and temperature changes under downhole producing conditions. The flexibility of cement may further be improved by reducing its density. However, in deep, hot, high-pressure wells, this option is not available since high-density cements are required for well security and control. Unfortunately, silica is difficult to mix with cementitious slurry to render high-density cements. Since silica is 25% lighter than cement, mix slurries above 16.0 lbm/gal (1.92 SG) and heavy weight agents, such as hematite or manganese oxide, are often required to increase the ratio of mix water needed to maintain the requisite mixability. Further, since silica is a solid material, pre-blending with the cement is required when used at high concentrations. A separate container is therefore required for storage purposes, typically necessitating the storage of both blended and non-blended cement at the wellsite. This becomes more important and expensive in remote locations, and especially in offshore locations where space on drilling rigs is quite limited. Recently, a commercial version of a slurried suspension of a mix of silica sand/flour/fume has become available in an effort to circumvent this problem. However, high volumes of the silica slurry are required. It is difficult therefore to justify the economics of its use.
A need exists therefore for cementitious slurries capable of counteracting strength retrogression, especially when used at low concentrations. Such materials should be effective at concentrations less than 5% BWOC. For high-pressure high temperature applications, the material should also have a higher specific gravity than cement. Further, it is desired that such new cementitious slurries be more flexible than those slurries of the prior art which incorporated silica sand and silica flour.